Technical Field
The invention relates to a sensing unit with at least one sensor element, the matrix and sensor connections (indicators) of which are protected from damaging or inactivating influences such as highly reactive compounds by means of at least one other protector element arranged on the media side. The invention further relates to a sensor comprising a sensing unit of this type and to the use of the sensing unit and the sensor for detecting an analyte in an environment that is aggressive for the sensor element.
Discussion of Art
Sensor assemblies are generally known, the measurement principle of which is based on the fact that the sensory-active compounds (indicators) arranged in the sensor or in its sensory element are converted to an excited energy state by the supply of excitation energy. When they release energy, e.g., in the form of light of a certain wavelength, the indicators switch to a lower energy level.
The determination of an analyte in a sample is usually performed by measuring the energy emitted by the indicators, which are sufficiently changed upon contact with an analyte to permit detection.
Different sensor types can be distinguished based on the form of the excitation and emission energy. The indicators of optical sensors, for example, can be excited by the supply of light or chemical or electrical energy, while in each case the emission takes place in the form of light of a definite wavelength. In purely optical sensors, the excitation and emission of the indicators occurs in each case in the form of light of a specific excitation wavelength (v1) and emission wavelength (v2). Optical sensors are used, for example, to determine oxygen, halide ions, heavy metal ions, carbon dioxide (CO2), and the pH value. Here, the sensor principle is based on the characteristic of the indicators to change their optical properties upon contact with the analyte to be determined. The analyte can be detected, for example, by measuring the change of wavelength, the intensity of the emitted light or the luminescence quenching, the change of the luminescence decay time, or the relaxation time of the excited states of the indicator molecules, the phase shift between modulated excitation light and emitted light, and/or the absorption of light waves.
The sensory element of known optical sensors includes at least one sensor element, in or on which the indicators are arranged, such that the indicators are mostly immobilized or incorporated at the surface of the sensor element facing toward the medium being examined. The matrix material of the sensor element is made, for example, from polymer compounds, which may be doped with the indicators.
Sensor elements are generally designed as membranes. Furthermore, state-of-the-art optical sensors generally include a carrier element, which serves as a substrate for the sensor element and may be designed to be transparent to the excitation and/or emission light of the indicators. To achieve high measuring sensitivity, materials with a high transparency such as clear transparent plastics, clear glass or glass fibers are used for the carrier element. Optical sensors also generally include a measuring technique that permits the optical detection of the above-mentioned changes in the optical indicators due to interaction with the analyte being detected. For the measurement it is generally necessary to transmit light through the carrier element to the indicator molecules and to receive it from these again.
At present, sensors are used for the optical measurement of dissolved oxygen which contain luminescence indicators, the luminescence of which is excited by light irradiation of a specific wavelength v1, has itself a wavelength v2, and is dynamically quenched in the presence of oxygen, and the excited state of the luminescence indicator is deactivated in a radiationless manner by oxygen.
Optical sensors are widely used for the reliable determination of an analyte such as molecular oxygen in complex media, because the measurement methods used here are relatively simple and entail low equipment costs. However, conventional optical sensors have the disadvantage that their sensor elements do not provide adequate or long-lasting protection of the indicators they contain from destructive influences, in particular from reactive compounds in the environment being analyzed or from high temperatures. The service life of known sensor elements is therefore especially limited when the individual sensor components, in particular the indicators, are exposed to conditions under which they are permanently and irreversibly damaged or inactivated.
In various applications, the medium to be analyzed contains, for example, compounds which can destroy the indicators due to a chemical reaction, if they are not adequately protected. Thus, the problem with the current state of the technology is that optical sensor elements currently available for determining molecular oxygen either cannot be used or only with a very limited service life, if the medium to be analyzed, such as waste water or water from swimming pools, contains strong oxidants such as ozone, superoxides, or hydroxyl radicals or chlorine or peroxide compounds used for disinfection, the diffusion of which to the indicators in the sensor element cannot be prevented, and contact with them leads to the oxidative inactivation of the indicators. Due to a lack of protection or inadequate protection of the indicators, conventional sensor elements are either not suitable or only to a very limited degree under these conditions.
Optical sensors are still used in areas where methods of analysis must be performed under sterile conditions. Such areas include in particular medicine, the food industry, and biotechnology, which primarily use disposable systems at present. A particular challenge is to provide (optical) sensing units or (optical) sensors which can be sterilized, especially heat-sterilized, because it is above all (optical) sensors which can be heat-sterilized, for example steam-sterilized, which would have economic and ecological advantages over disposable systems, due to their reusability in the sterile environment. However, the optical sensors currently known from the prior art cannot be exposed to the conditions of heat sterilization or only to a very limited extent, since their structure and especially the connectivity of the individual components would be damaged under such conditions, as a result of which sensitive elements of the sensor would be exposed to a destructive environment and the correct functioning of the sensor would thus no longer be ensured.
Heat-sterilizable sensors have to meet special requirements. In particular, the sensor element, including indicators and a (polymeric) matrix for immobilizing them, must be composed in such a way that permeability to the analyte and transparency to the excitation light and the luminescent light is sufficiently retained even after the application of heat during sterilization (e.g. autoclaving). The decisive factor for achieving this is the thermal resistance of the materials used. In addition, the structure, in particular the arrangement and connection of the individual sensor elements in the sensor is essential for the provision of temperature-stable sensors. Therefore it is does not help that the matrix of the sensor element is selected from a heat-stable polymer, if during the sterilization process the sensor suffers mechanical damage, for example, which prevents its continued use according to its intended purpose.
In the optical sensors currently used in process measurement technology, it was found that the indicators they contain are no longer adequately protected against chemical attack, particularly after thermal loads such as those occurring in the course of cleaning and sterilization processes, e.g. during autoclaving, during CIP (Clean in Place) and SIP (Sterilize in Place) treatments. Damage to the indicator-bearing elements mainly occurs when certain substances are present in the liquid process medium and/or cleaning agent, which on reuse of the once heat-sterilized sensor can penetrate its polymeric indicator-bearing sensor element membrane. This is due, on the one hand, to the matrix membrane polymer used for the sensor elements and, on the other hand, to the construction or design of the sensor. The latter can be damaged, for example, due to the influence of heat during the cleaning or sterilization process, which results is the passage of aggressive substances through to the sensing element.
There is therefore a need for sensors or sensing units, in particular optical sensors or sensing units for the luminescence detection of analytes in complex media, the sensor components of which, and in particular the luminescence indicators, are effectively protected against destructive or inactivating influences such as reactive chemicals. This type of sensor or sensing unit should preferably be sterilizable, and especially heat-sterilizable.
Optical sensors whose sensor element matrix consists of thermally stable polymers, are known, e.g., from WO2009016236A1. Mentioned here are those polymers that have a non-aromatic backbone chain, i.e., cyclic olefin polymers or cyclic olefin copolymers such as ethylene-norbornene copolymer, poly (n-methyl methacrylimide) or mixtures of these. A disadvantage of this solution is that the number of matrix polymers, already restricted due to the measuring principles applicable to optical sensors, is further restricted here. As a result, among other things, the luminescence response of a luminescence indicator is strongly dependent on the polymer matrix used. The previously used electronics and firmware of known sensors would have to be adapted to new parameters corresponding to these polymers.
A similar solution proposal for improving the chemical stability of the sensor element membrane of optical sensors is disclosed in EP 1199556B1. This reveals a fluoropolymer as the matrix polymer. In a matrix containing fluoropolymers, as explained in detail in this document, only certain transition metal complexes with special, at least partially fluorinated, ligands, may be employed as luminescent indicators, which severely restricts the application range of the sensors. For example, compounds which are commercially available as oxygen luminescence indicators and commonly used, e.g. platinum porphyrin complexes, cannot be used in a matrix of this type.
It is known from CH 677151 and EP 0478720B1 that films, foils, or membranes made from fluoropolymers, particularly PTFE, can be used to increase the chemical stability of a downstream layer or membrane of an electrochemical or amperometric sensor.
Optical sensors, especially optical sensors for oxygen analysis, the indicator-bearing sensor elements of which are protected against the attack of reactive compounds even after sterilization, thus permitting the long-lasting determination of the analyte by means of luminescence measurement in a (chemical) environment that is aggressive for the indicators, are not known from the prior art, nor can they be derived from it.